A detailed characterization of the electrical behavior of a K ion-conducting membrane protein from sacroplasmic reticulum of mammalian skeletal muscle is proposed. The study is carried out by measuring currents flowing across planar phospholipid bilayer membranes into which these channels have been inserted. Questions to be addressed are: (1) how does the channel undergo transitions between its conducting and nonconducting states, and (2) what is the basis of its ability to discriminate among different cations? Ultimately, the channel protein will be purified in a functionally active state.